First of all, thank you for this project, brings new life to DF and it can unleash the true potential of the game!

Now, I always thought that having litterally thousands of dungeons at disposal but being able to grind/farm in just one by exiting and reentering was just unimmersive... I think with not-respawning dungeons there'd be also a sense of accomplishment in each playtrough, and quests should only point you toward unvisited dungeons...

Is it doable?

Also I would say that this would be perfect paired with an unleveled world! What do you think?

What you're suggesting is that the game keeps full state for every dungeon you visit. I think that's a bit much to ask for a game the size of Daggerfall, so no.

The reset is so you can't gradually grind through a dungeon, once you're in you complete the mission or you exit and are back to square one. It would be easy enough to have dungeon state persist until you entered a different dungeon, so you could exit and return to pick up exactly where you left off, but this would break that part of the DF game design so I didn't implement it when I did the work persistence code. It could be added as a toggle behaviour switch if a lot of people were keen, but my opinion was it broke a fundamental part of DF, and I would need quite some convincing.

I see, a lot of data to store. Anyway I don't know if I've been misunderstood but I was thinking about this as a mod, not a in-game feature of the base game. My bad if I wasn't clear enough.

Yeah probably the fact that you can't reset the dungeon anymore upon reentering opens other scenarios and yoy can even break the game, but it's normal with modding and you must take everything into consideration.

Just out of curiosity... how difficult is DU modding, say, compared to Morrowind? (I only had modding experience with that game)

There are other options worth considering than remembering the state of all dungeons or remembering the state of only the last dungeon.

Something along the lines of saving dungeon state for X in-game days would seem reasonable both in terms of gameplay experience and technical requirements (tbh even saving dungeon state for a few 1000 dungeons in environments as simple as df should be well within reason in the $currentyear).

Truth be told, remembering what you have been in is simple and lightweight. 0 for not been there, 1 for been there. Its exactly as many bits of information as there are dungeons. Shoehorning this data into the quest system is not as easy, I am sure, and you would have to have some method of dealing with a character that has been in every dungeon in the province. That could also be problematic. So overall, the mod design would look like this:

1) Change dungeon respawn. For simplicity just respawn only the enemies that have not been killed, which would allow you to simply keep a binary dead/not dead flag for each creature in a small table per dungeon.

2) Have the visited status of each dungeon tracked, again binary flag for visited/not visited.

3) Get quest system to take into consideration this table when generating quests.

Steps 1 and 2 are super simply (in theory, I have never looked at the code, simply speaking in broad programming terms). 3 is where the beast lies, and I don't think that this can be done as a mod, as it seems unlikely the bits and bobs we would need to manipulate would be available to grab hold of. Again, not looked at this code, just throwing out some basic suggestions. Could even work up a pseudo code example if someone wanted. I do not have time nor energy to be involved in any more hands on kind of way however.

"Dagoth Ur is dead. I hope we will no longer be troubled by his dreams. But I wonder, too, what the ghost of a god would be. And can a dead god dream?"

Truth be told, remembering what you have been in is simple and lightweight. 0 for not been there, 1 for been there. Its exactly as many bits of information as there are dungeons.

It would take a bit more work than that. It's not just a matter of remembering where player has or has not visited, and you can't just assume player has or has not cleared a dungeon. You also need to save the state of dungeons visited for this to be truly meaningful. For example, which enemies are alive or dead? If dead, where did their corpse drop? If alive, how many hit points do they have remaining? What about spell points? Have they moved from their start location? Are any spell effects active on them? How long will those effects run for? Are doors open or closed? Are there any treasure piles? What items do these piles contain? Did the player drop any items on the ground? If the player dropped an item, what condition is it in? And so on. Multiply that across hundreds or thousands of dungeons and things get out of hand very quickly.

Daggerfall Unity could potentially save this state on a per-dungeon basis thanks to Hazelnut's excellent world persistence layer over my save game system. We even discussed the idea of serializing dungeon state like buildings, but rejected this as not only would it require substantial amounts of data, it also changes Daggerfall's design. Dungeon resetting is, for better or worse, part of Daggerfall's core game design. And as you correctly surmised, quests add a wrinkle across everything as they assume dungeons have volatile state.

It would definitely make an interesting mod and most of the moving parts are already in the game. The mod author would need to set some rational limits on how many dungeons worth of state are stored before culling and allowing dungeon to repopulate. They should also be careful not to pollute core save data in case player decides to remove mod later. Not sure how this would integrate with quest system either, it feels like it would create potential for broken quests. So loads of engineering concerns if anyone decides to tackle this someday.